1. Field of the Invention
The invention relates to a device to control the output power of a class C amplifier and, especially, an amplifier of the type using bipolar transistors.
2. Description of the Prior Art
The uses of class C power amplifiers include the amplification of pulses of the type used by radars, namely pulses with carrier frequency. For a wave with a given frequency, the essential requirements for these power amplifiers are:
a constant output power;
a constant input power;
a constant supply voltage;
a defined pulse rising time;
a defined drop in power during the pulse.
These five requirements cannot be met simultaneously in the case of power amplifiers that use bipolar transistors which work in class C mode. For, in this case, there is a ripple in the output power owing to mismatching at the output. The result of this is that, when amplifiers of this type are placed in series, the last amplifier has input power levels that are far too dispersed in the frequency band to be capable of functioning efficiently.
To overcome this drawback, amplifiers have been made with a variable supply voltage or a variable input power. To obtain a variable supply voltage, action is taken on the supply voltage Vcc of the bipolar transistor collector to control the level of the output power: the voltage is obtained through a battery of capacitors and, consequently, the variation does not occur immediately between two pulses, especially if they are separated by very short intervals of about a few microseconds. Furthermore, a variation in the supply voltage is accompanied by a variation in phase of a few degrees per volt, and this cannot be tolerated in certain applications, especially in pulse compression type radars.
To obtain a variable input power, an attenuator controlled by information on the output power is inserted in the amplification chain. A method of this type has the following disadvantages:
the insertion loss is high, in the range of a few decibels;
the cost of the attenuator is high when high peak and mean power values have to be dissipated;
the output pulse is distorted, especially at its start, because there is a delay between the information on the power and the attenuator effect.